Acceleration of oxidation processes over vanadia catalysts

ABSTRACT

A gaseous mixture comprising an oxidizable substance and oxygen is passed over a vanadia catalyst while one or more of these reaction conditions is cycled: A. RATIO OF OXIDIZABLE SUBSTANCE TO OXYGEN, B. SUM OF THE PARTIAL PRESSURES OF OXIDIZABLE SUBSTANCE AND OXYGEN, C. THE FLOW RATE OF THE GASEOUS MIXTURE, D. THE TEMPERATURE AT WHICH THE GASEOUS MIXTURE IS PASSED TO THE CATALYST, AND E. THE TEMPERATURE OF THE CATALYST ITSELF; THE CYCLES ARE OF ABOUT TWO TO ABOUT EIGHT HOURS DURATION. Under these conditions the rate of reaction is significantly greater than the rate under steady-state conditions normally used.

United States Patent [191 Silveston et al.

[451 Dec.24, 1974 ACCELERATION OF OXIDATION PROCESSES OVER VANADIA CATALYSTS [22] Filed: July 12, 1972 [21] Appl. No.: 270,933

[52] US. Cl. 423/533, 423/532 [51] Int. Cl. C0lb 17/76, COlb 17/68 [58] Field of Search 423/538, 535, 534, 533, 423/532, 244, 242; 260/687 [56] References Cited UNITED STATES PATENTS 1,366,439 1/1921 Weber 423/538 3,072,457 1/1963 Bloch 423/214 X 3,312,529 4/1967 Evano 423/576 X 3,424,560 l/l969 Carmassi et al. 423/567 X 3,454,356 7/1969 Raman 423/535 X FOREIGN PATENTS OR APPLICATIONS 258,974 10/1926 Great Britain 423/533 753 3/1871 Great Britain 423/533 4/1960 Great Britain .4 423/532 3/1936 Great Britain 423/533 Primary ExaminerOscar R. Vertiz Assistant ExaminerGary P. Straub Attorney, Agent, or Firm-A. H. Heatley [57] ABSTRACT A gaseous mixture comprising an oxidizable substance and oxygen is passed over a vanadia catalyst while one or more of these reaction conditions is cycled:

a. ratio of oxidizable substance to oxygen,

b. sum of the partial pressures of oxidizable substance and oxygen,

c. the flow rate of the gaseous mixture,

(1. the temperature at which the gaseous mixture is passed to the catalyst, and

e. the temperature of the catalyst itself;

the cycles are of about two to about eight hours duration. Under these conditions the rate of reaction is significantly greater than the rate under steady-state conditions normally used.

2 Claims, No Drawings ACCELERATION OF OXIDATION PROCESSES OVER VANADIA CATALYSTS This invention relates to oxidation processes which use vanadium pentoxide catalysts. Examples of such processes are the partial oxidation of naphthalene, mixed xylenes or o-xylene to phthalic anhydride, benzene to maleic anhydride, and the oxidation of sulfur dioxide to sulfur trioxide for the production of sulphuric acid. However, the invention is not limited to these four specific processes.

Processes for the production of phthalic anhydride are well known in which molten naphthalene is vaporized by bubbling preheated air through the melt, and additional air is added to bring the air-naphthalene ratio to 18 to 22:1 by weight. The vapor is passed over the catalyst at a temperature of about 650 to 850F. The catalyst is contained inside tubes immersed in mercury or a molten salt which serves to remove the heat of reaction. Phthalic anhydride and by product maleic anhydride are recovered in condenser banks located after a gas cooler. The support for the potassium sulfate promoted vanadia catalyst may be either silica gel or alumina. Yields of 70 to 80 percent are normal with essentially complete oxidation of the naphthalene. O- xylene or mixed xylenes may be substituted for naphthalene. Maleic anhydride may be obtained from benzene using the same process.

Processes for the production of sulfuric acid are well known in which elemental sulfur is burned in an excess of air to produce a gaseous reaction mixture consisting essentially of about 7 to 12 percent sulfur dioxide, about to 14 percent oxygen, and the remainder nitrogen. The reaction mixture is cooled to a temperature of about 410 to 460C. usually in a heat exchanger producing steam, and passed over catalyst beds in several stages, usually with cooling and/or absorption of sulfur trioxide in sulfuric acid between stages. The catalyst may be vanadium pentoxide or platinum or a platinum group metal supported on a suitable carrier. In the final stages the sulfur dioxide is essentially completely converted to sulfur trioxide which is absorbed in sulfuric acid. Alternatively, the sulfur dioxide may be obtained from various manufacturing processes, for example from the waste gases of metallurgical smelters, processed to obtain a gaseous reaction mixture with suitable concentrations of sulfur dioxide, oxygen and diluent.

In these well known processes the standard practice is to conduct the process with the various conditions held constant within the narrowest practical limits. We have now found that when the catalyst is vanadium pentoxide, which may be promoted or not, the average rate of the above noted reactions is increased by varying one of more of the reaction conditions:

a. the ratio of the partial pressures of the oxidizable substance to oxygen,

b. the sum of the said partial pressures,

c. the flow rate of the reaction gas mixture,

d. the temperature at which the reaction gas mixture is passed to the catalyst, and

e. the temperature of the catalyst.

The variation is carried out in cycles ranging from about 2 hours to about 8 hours; cycles in the range of limit of the reaction condition to the other limit is made abruptly, or it may have a wave form approximating a sine curve, where the variation is smooth and continuous.

The effectiveness of vanadium pentoxide to increase the rate of oxidation under these variations is believed to be due to its unusual nature. The oxygen in its lattice is mobile and capable of re-oxidizing surface sites where oxidation of the reactant is occurring. However our invention is not limited by the theory of the mechanism of the reaction.

Theoretical studies by J. M. Douglas (I. and EC. Proc. Des. and Dev. 6, 43 (1967)) have indicated that in the case of a single reactant the capacity of a reactor could be increased by cycling the feed rate. His study did not include the effect of cycle periods or the extent of the variations.

In their experiments on the catalytic dehydrogenation of ethyl alcohol Denis and Kabel (Chem. Eng. Sci. 25, 1057 (1970) and A.I.Ch.E..l. 16, 972 (1970)) found that the reaction rate was lower when the flow ratewas cycled than when it was steady.

The process of the invention is illustrated but not limited by the following examples.

The reactor consisted of a stainless steel tube, one quarter inch inside diameter, submerged in a bed of fluidized sand maintained at a constant temperature. In the reactor was a single bed of vanadia catalyst weighing 0.31 grams, the particle size of which was very small, about 0.5 mm., to avoid diffusional interference. Appropriate control valves and calibrated manometers were arranged to feed dry nitrogen, sulfur dioxide, and oxygen, through a mixing manifold and a preheater to the reactor. The control valves were set to provide a constant flow of nitrogen and a constant flow of sulfur dioxide plus oxygen. For half of a selected time cycle the sulfur dioxide/oxygen ratio was held at a selected value; for the other half of the cycle the ratio was held at a different but constant value. The exit gas during each cycle, accurately timed, was absorbed in a solvent at 72C, which was later analyzed for sulfur trioxide by a standard procedure accurate to about 1 percent. The average rate of reaction for the cycle was expressed as gram moles of sulfur trioxide produced per hour per gram of catalyst.

In the first example, the temperature of the sand bath was held at 405C. The total gas flow was held at 41.3 grams per hour, and the pressure was about one atmosphere, and the mol ratio (i.e. the ratio of the partial pressures) of sulfur dioxide to oxygen was held at a constant value of 0.6 for a few days to establish the rate of reaction at this ratio. The ratio was then raised to and held at 0.9 for a period of time, after which it was lowered to and held at 0.3 for an equal period of time, while maintaining the sum of the partial pressures of the reactants at its original value of about 0.32 atm. This cycle was repeated about 15 times. The rates of reaction were reproducible within close limits; the average is reported.

In these examples the high-level and low-level ratios of 0.3 and 0.9 respectively are variations of 0.3 from the average ratio, 0.6. The observations are shown in Table l.

TABLE 1 As noted above cyclic variations in the ratio of oxidizable substance to oxygen significantly increase the Example cycle Rate reaction rate. We have also found that cyclic variations No: Period x to in the sum of the partial pressures of the reactants, even 5 when the said ratio is kept constant, significantly inla Steady Sum 31 crease the reaction rate. However, the simultaneous cylb 40 3.1 cling of the said ratio and the said sum of the partial {a 23 2:? pressures of the reactants is also effective to achieve a le 240 3.75 significant increase in reaction rate and lies within the $38 ambient of our invention. For example, this simultae neous ambit may be achieved by cycling the partial Gram moles of sulfur trioxide produced per hour per gram of catalyst. PRE SURE f Only n f the reaCtantS. On the other In the second example, carried out in the same reachand h Fame] pressures of both reactants e tor, the temperature, the total pressure, the total flow 1 5 cycled "P h Same e 'e and by the m factor rate, the sum of the pressures of the reactants, and the 2 K e to be f f g g the e average mol ratio of sulfur dioxide to oxygen were held t 6 e pressure? e re era y t 8 0 at the same values as in Example 1. In the successive t P pressufres g 2 T e fi parts of the example the cycle period was held constant the range a a a an e at 240 minutes and there were different sets of highposltlle g vef arlatons 0rgm t g satioajvelragge level and low-level ratios. The data are in Table 2. are w'thmt 6 range mm out to a re errably the sum of the partial pressures of the reactants TABLE 2 is varied from an average value within the range from 0.18 to 3.0 atm. and the positive and negative variag l L L h L I e g tions from the said average are within the range from eve lg eve X about 0.04 to about 0.6 atm. A numerical example is given in Table 4 to illustrate but not to limit the three 2a 0.6 0.6 Zero 3.1 Cases. 2b 0.5 0.7 0.1 3.1 20 0.4 0.8 0.2 3.52 2d 0.3 09 0.3 3.93 a. the ratio of the partial pressures 1S cycled while 2e 0.2 1, 4 181 their sum 18 kept constant,

b. the ratio is kept constant while the sum is cycled, Gram moles of sulfur trioxide produced per hour per gram of catalyst. and

In the third example, also carried out in the same rec. both ratio and sum are cycled. actor, the temperature, the total pressure, the total flow In all three cases the total pressure is one atmosphere.

' TABLE 4 Three cases of cycling. Average Value Low-level value High-level value Partial Partial Partial Pressure Ratio Pressure Ratio Pressure Ratio (atm.) (atm.) (atm.)

Case A. Ratio Sum cycled. constant. Oxidizable Subs. 0.12 0.6 0.074 0.3 0.152 0.9 Org gen 0.20 0.246 0. l 68 Case B Ratio Sum constant. cycled. Oxidizable Subs. 0.12 0.6 0.096 0.6 0.144 0.6 Ox gen 0.20 0.l6 0.24

S UM 0.32 0.256 0.384

Case C. Both ratio and sum cycled. Oxidizable Subs. 0.12 0.6 0.06 0.3 0. l 8 0.9 Oxygen 0.20 0.20 0.20 SUM 0.32 0.26 0.38

rate and the sum of the partial pressures of the reac- The rate of reaction is likewise increased significantly tants were held at the same values as in the preceding when the flow rate is varied in a cyclic manner over the examples. The cycle period was 240 minutes. Three rate when the flow is maintained constant. Preferrably different average mol ratios were used. The data are in the average rate is in the range from about 20 to about Table 3. 200 pounds of the reaction gas mixture per hour per pound of catalyst and the positive and negative varia- TABLE 3 tions are within the range from about 4 to about 50. Example Mean MOI Ratios variations Ram Cyclic variations in the temperature at which the gas N0: Low- Highx 10 15 passed to the catalyst or m the temperature of the Level Level 5 catalyst itself also significantly increase the reaction 38 0 3 0 2 0.4 0.1 13 rate over that obtained with a constant temperature. 3b 0.6 0.3 0.9 0.3 3.9 Obviously these variations must not take the temperature downward to values where the normal catalytic ac- Gram moles of sulfur trioxide produced per hour per gram of catalyst.

tivity is significantly diminished, nor upward to values where equilibrium considerations have a significant adverse effect. Preferrably the average temperature at which the gas is passed to the catalyst is within the range from 380C. to 550C. and the positive and negative variations from the said average are within the range from about 5C to about 50C. Preferrably the average temperature of the catalyst is within the range from about 420C to about 600C and the positive and negative variations from the same average are within the range from about 5C to about 25C.

What is claimed is:

l. In a process for producing sulfur trioxide which comprises passing a gaseous reaction mixture compris- 2. The improvement of claim ll, wherein the said cycles are of about 4 to 6 hours duration.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,856,927 Dated 24th December 1974 Inventor) Peter L. Silveston, Robert R. Hudgins It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 50 after "various" insert reaction Column 4 line 10 alter "ambient" to ambit line 11 alter "ambit" to cycling .Si gned and Sealed this 7th day of June. 19 5.

C. ZiAFiPJEZS-QL L'AE'FN CUTE 1'. "i-C-i Commissioner of k'atents l I I?! Q 1 -.ttest;.m; Of 2 1 Gel and lraoemaiica ORM PO-XOSO (10-69) USCOMM-DC eoan-Pae U45. GOVERNMENT PRINTING OFFICE l9. O-lil- 

1. IN A PROCESS FOR PRODUCING SULFUR TRIOXIDE WHICH COMPRISES PASSING A GASEOUS REACTION MIXTURE COMPRISING OXYGEN AND SULFUR DIOXIDE OVER VANADIUM PENTOXIDE CATALYST, THE IMPROVEMENT WHICH COMPRISES: CARRYING OUT THE REACTION WHILE THE RATIO OF PARTIAL PRESSURE OF SULFUR DIOXIDE TO OXYGEN IS VARIED IN CYCLES FROM A VALUE SELECTED FROM THE RANGE OF ABOUT 0.6 TO ABOUT 1.1 AND THE POSITIVE AND NEGATIVE VARIATIONS FROM SAID VALUE ARE WITHIN THE RANGE FROM ABOUT 0.2 TO ABOUT 0.5 AND ARE OF EQUAL MAGNITUDE, SAID CYCLES BEING OF ABOUT 2 TO 8 HOURS DURATION.
 2. The improvement of claim 1, wherein the said cycles are of about 4 to 6 hours duration. 